A highly divergent member of the class I MHC gene family has been isolated from the genome of the C57BL/10 mouse. This gene, Mb1 is linked to the remaining members of the family, mapping to the D-end of the complex. Although Mb1 is not expressed in transfected mouse L cells, it is expressed in vivo in a variety of tissues, with preferential expression in T cells. Furthermore, it is expressed in high levels in a T cell lymphoma. DNA sequence analysis reveals that the exon organization of this gene resembles that of other class I genes. However, the level of DNA sequence hemology of Mb1 to other H2 genes is no greater than to human HLA genes. Furthermore, whereas all previously reported H2 class I genes have third introns of 1.2-2 kb, Mb1 has an intron of 600 bp, similar to those of human and porcine class I genes. Taken together, these data suggest that Mb1 may represent a direct descendant of a primordial class I gene, which antedates speciation.